Mems varactors

ABSTRACT

MEMS varactors capable of handling large signals and/or achieving a high capacitance tuning range are described. In an exemplary design, a MEMS varactor includes (i) a first bottom plate electrically coupled to a first terminal receiving an input signal, (ii) a second bottom plate electrically coupled to a second terminal receiving a DC voltage, and (iii) a top plate formed over the first and second bottom plates and electrically coupled to a third terminal. The DC voltage causes the top plate to mechanically move and vary the capacitance observed by the input signal. In another exemplary design, a MEMS varactor includes first, second and third plates formed on over one another and electrically coupled to first, second and third terminals, respectively. First and second DC voltages may be applied to the first and third terminals, respectively. An input signal may be passed between the first and second terminals.

BACKGROUND

I. Field

The present disclosure relates generally to electronics, and morespecifically to micro-electro-mechanical system (MEMS) varactors.

II. Background

MEMS is a technology used to form miniature electro-mechanical deviceswith mechanical moving parts. These devices may be used to implementvarious radio frequency (RF) circuit components such as variablecapacitors (varactors), switches, resonators, inductors, etc. MEMSdevices may have certain advantages over RF circuit componentsfabricated in other manners, such as higher quality factor (Q), lowerinsertion loss, better linearity, etc.

A MEMS varactor typically includes two terminals or electrodes. Oneterminal is typically used for a common terminal, which may be forcircuit ground or some other common connection. The other terminal maybe used for both an RF signal and a direct current (DC) voltage. The DCvoltage may be varied to mechanically move a plate within the MEMSvaractor, which may then adjust the capacitance of the MEMS varactor.The RF signal may be passed through the MEMS varactor and may have itscharacteristics (e.g., frequency, amplitude, etc.) altered by thecapacitance of the MEMS varactor.

The 2-terminal MEMS varactor described above may be used for a low-powerapplication with a small RF signal. In this case, the capacitance of theMEMS varactor may not be varied too much by the RF signal. However, theRF signal may be relatively large for a high-power application, such asa transmitter of a wireless communication device. If a large RF signalis applied to the MEMS varactor, then the capacitance of the MEMSvaractor may be varied by a large amount due to a large root mean square(RMS) voltage of the RF signal, which may be undesirable. A MEMSvaractor that can handle a large RF signal, with little or acceptablechanges in capacitance due to the large RF signal, would be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show a MEMS varactor with three terminals.

FIGS. 3A to 3D show different operational modes of the MEMS varactor in

FIGS. 1 and 2.

FIGS. 4 and 5 show another MEMS varactor with three terminals.

FIGS. 6A to 6D show different schemes for controlling changes to thecapacitance of a MEMS varactor for a large RF signal.

FIGS. 7 and 8 show two MEMS varactors with vertically stacked plates.

FIG. 9 shows a block diagram of a wireless communication device.

FIG. 10 shows a notch filter implemented with MEMS varactors.

FIG. 11 shows a process for operating a MEMS varactor.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any design described herein as “exemplary”is not necessarily to be construed as preferred or advantageous overother designs.

Various exemplary designs of MEMS varactors are described herein. TheseMEMS varactors may be used for various circuits such as tunable filters,tunable antennas, etc. Some of these MEMS varactors may be able tohandle large RF signals and may be used for high-power applications. Forexample, the MEMS varactors may be used for a transmitter of a wirelesscommunication device, which may be required to provide a large outputpower, e.g., 36 dBm for a power amplifier in GSM. The MEMS varactors maybe able to handle a large RF signal and may have a small change incapacitance due to the large RF signal. Some of the MEMS varactors mayalso be able to achieve a high capacitance tuning range.

In an aspect, MEMS varactors with three or more terminals may beimplemented with a horizontal structure. For a MEMS varactor with thehorizontal structure, bottom plates for an RF signal and a DC voltagemay be formed on the same level. A movable top plate may be formed overthe bottom plates and may be mechanically moved to vary the capacitanceof the MEMS varactor.

FIG. 1 shows a top view of an exemplary design of a MEMS varactor 100implemented with the horizontal structure. MEMS varactor 100 includesthree terminals. Different operating modes may be supported by applyingan RF signal and a DC voltage to the three terminals in differentmanners, as described below.

FIG. 2 shows a cross-sectional view of MEMS varactor 100 in FIG. 1. Thecross-sectional view in FIG. 2 is taken along line A-A′ in FIG. 1.

As shown in FIGS. 1 and 2, MEMS varactor 100 includes a center bottomplate 120 and side bottom plates 130 and 132 formed on top of asubstrate 110. Substrate 110 may be glass, silicon, or some othermaterial. Glass may have better performance as well as lower cost.Center bottom plate 120 may be formed in a metal layer or some otherconductive layer. Side bottom plates 130 and 132 may also be formed inthe metal layer or some other conductive layer. Center bottom plate 120and side bottom plates 130 and 132 may be formed in the same layer (asshown in FIG. 2) or in different layers. An insulation layer 140 may beformed over bottom plates 120, 130 and 132 with dielectric or some othernon-conductive material that can provide electrical insulation.

In the exemplary design shown in FIGS. 1 and 2, posts 142 and 144 may beformed over insulation layer 140 outside of bottom plates 130 and 132,respectively. In another exemplary design, bottom plates 130 and/or 132may extend underneath posts 142 and/or 144, respectively. In any case,posts 142 and 144 may be formed with oxide or some other material. Amoveable top plate 150 may be formed over posts 142 and 144 and may beseparated from bottom plates 120, 130 and 132 by a gap 152. Top plate150 may be formed with a conductive material and may also be referred toas a mechanical membrane, a mechanical electrode, etc.

As shown in FIG. 1, a first terminal 112 may be formed on one side ofcenter bottom plate 120. Side bottom plates 130 and 132 may be connectedby a conductor 136, and a second terminal 114 may be formed on conductor136. A third terminal 116 may be formed on one side of top plate 150.

MEMS varactor 100 operates as follows. A fixed DC voltage may be appliedto terminal 116. A variable DC voltage may be applied to terminal 112 or114. The voltage difference between the variable DC voltage applied toterminal 112 or 114 and the fixed DC voltage applied to terminal 116causes top plate 150 to move down. A large voltage difference wouldcause top plate 150 to move down more, which would then result in alarger capacitance for MEMS varactor 100. The converse would be true fora smaller voltage difference.

For example, terminal 116 may be coupled to circuit ground, and avariable DC voltage may be applied to terminal 112 or 114. A smallestcapacitance C_(min) may be obtained with zero Volts applied to terminal112 or 114, which would cause top plate 150 to rest at its normalposition that is farthest away from bottom plates 120, 130 and 132. Alargest capacitance C_(max) may be obtained with a sufficient voltageapplied to terminal 112 or 114, which would cause top plate 150 to movetoward bottom plates 120, 130 and 132 and rest on insulation layer 140.The voltage used to obtain C_(max) is referred to as a pull-in voltageV_(pull-in).

FIG. 3A shows a schematic diagram of MEMS varactor 100. Center bottomplate 120 is coupled to terminal 112, side bottom plates 130 and 132 areboth coupled to terminal 114, and top plate 150 is coupled to terminal116. An RF signal and a DC voltage may be applied to terminals 112 and114 in several manners, as described below.

FIG. 3B shows a first operational mode (mode 1) for MEMS varactor 100.In this mode, an RF signal is applied to terminal 112 coupled to centerbottom plate 120. A DC voltage is applied to terminal 114 coupled toside bottom plates 130 and 132. The first mode may be used to obtain alarge capacitance tuning range, i.e., a large C_(max)/C_(min).

FIG. 3C shows a second operational mode (mode 2) for MEMS varactor 100.In this mode, a DC voltage is applied to terminal 112 coupled to centerbottom plate 120. An RF signal is applied to terminal 114 coupled toside bottom plates 130 and 132. Movable top plate 150 is more stiff atthe two ends near posts 142 and 144 and is less stiff toward the centralarea. Thus, top plate 150 may move less when a large RF signal isapplied to side bottom plates 130 and 132, due to greater stiffness oftop plate 150 at the two ends. This may result in less change incapacitance due to the larger RF signal. Operating MEMS varactor 100 inthe second mode may result in less sensitivity to higher power (orhigher RMS voltage) of the RF signal.

FIG. 3D shows a third operational mode (mode 3) for MEMS varactor 100.In this mode, an RF signal and a DC voltage are both applied to terminal112 coupled to center bottom plate 120. Because movable top plate 150 isless stiff toward the central area, a smaller DC voltage may be used toobtain C_(max) in the third mode.

Table 1 summarizes the three operational modes for MEMS varactor 100.Table 1 also gives C_(min), C_(max), capacitance tuning range(C_(max)/C_(min)), and pull-in voltage V_(pull-in) for each of the threemodes for an exemplary design of MEMS varactor 100.

TABLE 1 Mode 1 Mode 2 Mode 3 RF signal Terminal 112 Terminal 114Terminal 112 applied to . . . DC voltage Terminal 114 Terminal 112Terminal 112 applied to . . . C_(min) (with 0.05 pF 0.10 pF 0.08 pF 0 Vapplied) C_(max) (with 0.91 pF 0.16 pF 0.94 pF V_(pull-in) applied)Capacitance 18.2 1.6 11.8 tuning range (C_(max)/C_(min)) Pull-in voltage16.0 V 8.5 V 6.2 V V_(pull-in)

In general, C_(min) and C_(max) may be dependent on the size of thebottom plate(s) to which the RF signal is applied. A larger capacitancemay be obtained with a larger plate size, and vice versa. C_(min) may beobtained with the top plate being farthest from the bottom plate(s) andmay be further dependent on the gap distance between the top plate andthe bottom plate(s). Smaller C_(min) may be obtained with a larger gap,and vice versa.

In the first mode, C_(min) and C_(max) may be determined by the size ofbottom plate 120, and C_(min) may be determined further by the gapdistance between bottom plate 120 and top plate 150. In the second mode,C_(min) and C_(max) may be determined mostly by the size of bottomplates 130 and 132. In the third mode, C_(min) and C_(max) may bedetermined by the size of bottom plate 120, and C_(min) may bedetermined further by the gap distance between bottom plate 120 and topplate 150. The desired C_(min) and C_(max) may be obtained (i) with anappropriate size for the bottom plate(s) to which the RF signal isapplied, (ii) with an appropriate gap distance between the bottomplate(s) and the top plate, and/or (iii) by varying othercharacteristics or parameters of MEMS varactor 100.

As shown in Table 1, the first mode may be well suited forbinary/digital applications, e.g., with the RF signal being switched onand off. The second mode may be well suited for high-power applications.The third mode may be well suited for low-bias applications.

FIG. 4 shows a top view of an exemplary design of a MEMS varactor 400with three terminals.

FIG. 5 shows a cross-sectional view of MEMS varactor 400 in FIG. 4. Thecross-sectional view in FIG. 5 is taken along line A-A′ in FIG. 4.

As shown in FIGS. 4 and 5, MEMS varactor 400 includes a first set ofbottom plates 420, 422 and 424 and a second set of bottom plates 430 and432 formed on top of a substrate 410. Substrate 410 may be glass or someother material. Bottom plates 420 to 432 may be formed in a metal layeror some other conductive layer. An insulation layer 440 may be formedover bottom plates 420 to 432 with a non-conductive material.

Posts 442 and 444 may be formed with oxide or some other material overinsulation layer 440 outside of bottom plates 420 and 424, respectively.A moveable top plate 450 may be formed with a conductive material overposts 442 and 444 and may be separated from bottom plates 420 to 432 bya gap 452. Top plate 450 may move down when a DC voltage is applied, maybe stiffer at the two ends near posts 442 and 444, and may be less stiffnear the central area.

As shown in FIG. 4, a conductor 428 may connect bottom plates 420, 422and 424 and may be coupled to a first terminal 412. A conductor 438 mayconnect bottom plates 430 and 432 and may be coupled to a secondterminal 414. A third terminal 416 may be formed on one side of topplate 450.

In a first mode, an RF signal may be applied to terminal 412, and a DCvoltage may be applied to terminal 414. In a second mode, the RF signalmay be applied to terminal 414, and the DC voltage may be applied toterminal 412. In a third mode, the RF signal and the DC voltage may bothbe applied to terminal 412. In a fourth mode, the RF signal and the DCvoltage may both be applied to terminal 414. Different varactorcharacteristics (e.g., C_(min), C_(max), capacitance tuning range, andV_(pull-in)) may be obtained for the four modes.

FIGS. 1 and 2 show an exemplary design of MEMS varactor 100 with threebottom plates 120, 130 and 132 formed under top plate 150. FIGS. 4 and 5show an exemplary design of MEMS varactor 400 with five bottom plates420, 422, 424, 430 and 432 formed under top plate 450. In general, anynumber of bottom plates may be formed under a top plate. More bottomplates may provide more freedom to obtain the desired varactorcharacteristics and may also allow for greater control of capacitancechange due to signal swing. The bottom plate(s) for the RF signal andthe bottom plate(s) for the DC voltage may be arranged in a comb-likestructure, as shown in FIGS. 1, 2, 4 and 5, or may be arranged in othermanners, e.g., with circular shape structures.

FIG. 6A shows an exemplary design for controlling changes in capacitancedue to electro-static force from a large RF signal applied to MEMSvaractor 400 in FIG. 4. In this exemplary design, the capacitance changemay be controlled by selecting suitable sizes for the bottom plates andexploiting the greater stiffness of the top plate near the two posts. Inparticular, less capacitance change due to the large RF signal may beobtained by (i) applying the RF signal to bottom plates 430 and 432,which are formed under a stiffer portion of top plate 450 than bottomplate 422, and (ii) forming bottom plates 430 and 432 with smaller areasthan bottom plates 420, 422 and 424 for the DC voltage.

FIG. 6B shows another exemplary design for controlling changes incapacitance due to electro-static force from a large RF signal appliedto MEMS varactor 400 in FIG. 4. In this exemplary design, thecapacitance change may be controlled by selecting suitable gap distancebetween the top plate and the bottom plates. In particular, lesscapacitance change due to the large RF signal may be obtained by having(i) a larger gap for bottom plates 430 and 432 to which the RF signal isapplied and (ii) a smaller gap for bottom plates 420, 422 and 424 towhich the DC voltage is applied. In general, a larger gap for the bottomplates for the RF signal may result in smaller capacitance change due tothe RF signal.

FIG. 6C shows an exemplary design for obtaining larger capacitanceand/or larger capacitance tuning range for MEMS varactor 400 in FIG. 4.In this exemplary design, the capacitance and/or capacitance tuningrange may be increased by having (i) a larger gap for bottom plates 420,422 and 424 to which the DC voltage is applied and (ii) a smaller gapfor bottom plates 430 and 432 to which the RF signal is applied.

FIG. 6D shows another exemplary design for controlling changes incapacitance due to electro-static force from a large RF signal. In thisexemplary design, a thin film resistor (TFR) 460 may be formed betweenbottom plate 420 and substrate 410. A thin film resistor 464 may beformed between bottom plate 424 and substrate 410. A thin film resistor462 may be formed on substrate 410 instead of bottom plate 422. Aplanarization layer 470 may be formed over bottom plates 420 to 432 withspin-coating techniques and may provide electrical isolation.

As shown in FIG. 6D, the gap may vary across top plate 450, with bottomplates progressively closer to the center of the top plate havingprogressively larger gap. This may decrease the DC pull-in voltage andimprove the tuning ratio.

In another aspect, MEMS varactors with three or more terminals may beimplemented with a vertical structure. For a MEMS varactor with thevertical structure, three (or possibly more) plates may be stackedvertically (i.e., placed in parallel) and coupled to three (or possiblymore) terminals. A middle plate may be mechanically moved to vary thecapacitance of the MEMS varactor.

FIG. 7 shows a cross-sectional view of an exemplary design of a MEMSvaractor 700 implemented with the vertical structure. MEMS varactor 700includes a bottom plate 720 formed on top of a substrate 710. Substrate710 may be glass or some other material. Bottom plates 720 may be formedin a metal layer or some other conductive layer. An insulation layer 722may be formed over bottom plate 720 with a non-conductive material. Amoveable middle plate 730 may be formed with a conductive material overbottom plate 720 and may be separated from the bottom plate by a gap732. A top plate 740 may be formed with a conductive material overmiddle plate 730 and may be separated from the middle plate by a gap734. An insulation layer 742 may be formed below top plate 740 with anon-conductive material.

In the exemplary design shown in FIG. 7, a terminal 712 may be formed onone end of bottom plate 720, a terminal 714 may be formed on one end ofmiddle plate 730, and a terminal 716 may be formed on one end of topplate 740. A first DC voltage V_(B1) may be applied via an RF choke 762(or a resistor) to terminal 712. An input RF signal (RFin) may beapplied via a DC blocking capacitor 764 to terminal 712. A second DCvoltage V_(B2) may be applied via an RF choke 766 (or a resistor) toterminal 716. An output RF signal (RFout) may be provided via terminal714.

Middle plate 730 may move up or down due to the DC voltages applied tobottom plate 720 and top plate 740. Insulation layers 722 and 742prevent middle plate 730 from shorting to bottom plate 720 or top plate740, respectively. A variable capacitor C1 may be formed between middleplate 730 and bottom plate 720. The capacitance of C1 may be determinedby the sizes of middle plate 730 and bottom plate 720 as well as the gapdistance between these two plates.

Top plate 740 may be used to compensate or reduce capacitance change dueto the input RF signal applied to bottom plate 720. A power detectionunit may measure the signal swing of the input RF signal. The second DCvoltage may be adjusted based on the measured RF signal swing. Forexample, a larger input RF signal may pull middle plate 730 towardbottom plate 720 and may increase the capacitance C1. A larger DCvoltage may then be applied to top plate 740 to pull middle plate 730toward top plate 740 and counter the pull by the larger input RF signal.

FIG. 8 shows a cross-sectional view of an exemplary design of a MEMSvaractor 800 implemented with the vertical structure. MEMS varactor 800includes a bottom plate 820 formed over a substrate 810, a movablemiddle plate 830, a top plate 840, and insulation layers 822 and 842, asdescribed above for MEMS varactor 700 in FIG. 7.

A terminal 812 may be formed on one end of bottom plate 820, a terminal814 may be formed on one end of middle plate 830, and a terminal 816 maybe formed on one end of top plate 840. A first DC voltage V_(B1) may beapplied via an RF choke 862 to terminal 812. An input RF signal may beapplied via a DC blocking capacitor 864 to terminal 814. A second DCvoltage V_(B2) may be applied via an RF choke 866 to terminal 716. Anoutput RF signal may be provided via DC blocking capacitors 872 and 874,which may be coupled to bottom plate 820 and top plate 840,respectively.

Middle plate 830 may move up or down due to the DC voltages applied tobottom plate 820 and top plate 840. A first variable capacitor C1 may beformed between middle plate 830 and bottom plate 820. A second variablecapacitor C2 may be formed between top plate 840 and middle plate 830.The capacitance of C1 may be determined by the sizes of plates 820 and830 as well as the gap distance between these two plates. Thecapacitance of C2 may be determined by the sizes of plates 830 and 840as well as the gap distance between these two plates. The totalcapacitance between the output RF signal and the input RF signal may begiven as C_(total)=C1+C2.

FIGS. 1 through 6B show some exemplary designs of MEMS varactors withthe horizontal structure. An RF signal may be applied to bottom plateslocated near the posts where the top plate has greater stiffness. Thismay result in smaller impact/deflection due to a large signal swing ofthe RF signal. Alternatively, the RF signal may be applied to bottomplates located away from the posts. This may result in a largercapacitance tuning range.

FIGS. 7 and 8 show some exemplary designs of MEMS varactors with thevertical structure. An RF signal may be applied to a bottom plate or amiddle plate of an MEMS varactor. A second DC voltage may be applied toa top plate formed over the movable plate to compensate for and reducecapacitance change due to a large signal swing of the RF signal.

The MEMS varactors described herein may provide certain advantages overconventional MEMS varactors. First, the MEMS varactors described hereinmay be able to handle a larger signal swing. This capability may beespecially beneficial for a high-power application such as a transmitterof a wireless communication device. Second, the capacitance tuning rangemay be controlled independently by a DC voltage with the horizontalstructure. A larger capacitance tuning range may be obtained with thevertical structure. For both the horizontal and vertical structures, thevaractor characteristics may also be controlled by selecting appropriateplate sizes, plate thickness, and gap distance, e.g., as described abovefor FIGS. 6A to 6D. The vertical structure may have other advantages,such as better thermal stability, which may be desirable for practicalproduct applications. Furthermore, varactor capacitance change due totemperature may be compensated with the top and the bottom plates. Incomparison to a varactor with a single electrode (which can onlyincrease the varactor capacitance), the vertical structure cancompensate for both capacitance increase/decrease due toincrease/decrease of temperature.

The MEMS varactors described herein may be used for various electronicsdevices such as wireless communication devices, cellular phones,personal digital assistants (PDAs), handheld devices, wireless modems,laptop computers, cordless phones, broadcast receivers, Bluetoothdevices, consumer electronics devices, etc. The use of the MEMSvaractors in a wireless communication device, which may be a cellularphone or some other device, is described below.

FIG. 9 shows a block diagram of an exemplary design of a wirelesscommunication device 900. In this exemplary design, wireless device 900includes a data processor 910 and a transceiver 920. Transceiver 920includes a transmitter 930 and a receiver 950 that supportbi-directional wireless communication. In general, wireless device 900may include any number of transmitters and any number of receivers forany number of communication systems and any number of frequency bands.

In the transmit path, data processor 910 processes data to betransmitted and provides an analog output signal to transmitter 930.Within transmitter 930, the analog output signal is amplified by anamplifier (Amp) 932, filtered by a lowpass filter 934 to remove imagescaused by digital-to-analog conversion, amplified by a variable gainamplifier (VGA) 936, and upconverted from baseband to RF by anupconverter 938. The upconverted signal is amplified by a poweramplifier (PA) 940, further filtered by a filter 942 to remove imagescaused by the frequency upconversion, routed through a duplexer/switch944, and transmitted via an antenna 946. Filter 942 may be implementedwith a MEMS notch filter that can handle high power from PA 940. Filter942 may be located after PA 940 (as shown in FIG. 9) or prior to PA 940.

In the receive path, antenna 946 receives signals from base stations andprovides a received signal, which is routed through duplexer/switch 944and provided to receiver 950. Within receiver 950, the received signalis amplified by a low noise amplifier (LNA) 952, filtered by a bandpassfilter 954, and downconverted from RF to baseband by a downconverter956. The downconverted signal is amplified by a VGA 958, filtered by alowpass filter 960, and amplified by an amplifier 962 to obtain ananalog input signal, which is provided to data processor 910.

FIG. 9 shows transmitter 930 and receiver 950 implementing adirect-conversion architecture, which frequency converts a signalbetween RF and baseband in one stage. Transmitter 930 and/or receiver950 may also implement a super-heterodyne architecture, which frequencyconverts a signal between RF and baseband in multiple stages. A localoscillator (LO) generator 970 generates and provides transmit LO signalsfor upconverter 938 and receive LO signals for downconverter 956. Aphase locked loop (PLL) 972 receives control information from dataprocessor 910 and provides control signals to LO generator 970 togenerate the transmit and receive LO signals at the proper frequencies.

FIG. 9 shows an exemplary transceiver design. In general, theconditioning of the signals in transmitter 930 and receiver 950 may beperformed by one or more stages of amplifier, filter, mixer, etc. Thesecircuits may be arranged differently from the configuration shown inFIG. 9. Furthermore, other circuits not shown in FIG. 9 may also be usedto condition the signals in the transmitter and receiver. Some circuitsin FIG. 9 may also be omitted. All or a portion of transceiver 920 maybe implemented on an analog integrated circuit (IC), an RF IC (RFIC), amixed-signal IC, etc.

Data processor 910 may perform various functions for wireless device900, e.g., processing for transmitted and received data. A memory 912may store program codes and data for data processor 910. Data processor910 may be implemented on one or more application specific integratedcircuits (ASICs) and/or other ICs.

As shown in FIG. 9, a transmitter and a receiver may include variousanalog circuits. Each analog circuit may be implemented in variousmanners and may include one or more MEMS varactors described herein. Forexample, MEMS varactors may be used in power amplifier 940, filter 942,duplexer/switch 944, LO generator 970, etc.

FIG. 10 shows a schematic diagram of an exemplary design of a notchfilter 1000 implemented with MEMS varactors. Notch filter 1000 may beused for filter 942 in FIG. 9. Within notch filter 1000, an input RFsignal (RFin) is provided to node A, and an output RF signal (RFout) isprovided via node C. A capacitor 1012 is coupled between node A andcircuit ground, and a capacitor 1014 is coupled between nodes A and B.An inductor 1016 and a MEMS varactor 1018 are coupled in parallelbetween node B and circuit ground and form a first resonator. Aninductor 1020 is coupled between nodes A and C. A capacitor 1022 iscoupled between node C and circuit ground, and a capacitor 1024 iscoupled between nodes C and D. An inductor 1026 and a MEMS varactor 1028are coupled in parallel between node D and circuit ground and form asecond resonator.

MEMS varactors 1018 and 1028 may each be implemented with MEMS varactor100, 400, 700 or 800 in FIG. 1, 4, 7 or 8, respectively. This may thenallow notch filter 1000 to handle a large input RF signal and providegood performance. For example, notch filter 1000 may be used for GSM,CDMA, WCDMA or some other cellular applications, and MEMS varactors 1018and 1028 may need to handle high RF power in a range of 28 to 35 dBm (or0.6 to 3.2 Watt), which may translate to a peak voltage in the range ofabout 30 to 75 V. In this case, the MEMS varactors should have a pull-involtage of greater than 30V for CDMA/WCDMA and greater than 75V for GSM.This may allow the capacitance of the MEMS varactor to be tuned mostlyby the DC voltage and to not be significantly influenced by a large RFsignal.

In an aspect, a MEMS varactor may comprise first and second bottomplates and a top plate. The first bottom plate (e.g., bottom plate 120or 130 in FIG. 1) may be electrically coupled to a first terminal, whichmay receive an input signal. The second bottom plate (e.g., bottom plate130 or 120) may be electrically coupled to a second terminal, which mayreceive a DC voltage. The top plate (e.g., top plate 150) may be formedover the first and second bottom plates and may be electrically coupledto a third terminal. The DC voltage may cause the top plate tomechanically move and vary the capacitance observed by the input signal.The input signal may be an RF signal or a signal of some other type. Theinput signal may have a large signal swing, e.g., of more than 10 Volts.

In an exemplary design, the MEMS varactor may further comprise a thirdbottom plate (e.g., bottom plate 132 in FIG. 1) formed under the topplate and electrically coupled to the first terminal. In this exemplarydesign, which may correspond to mode 2 in FIG. 3C, the input signal maybe applied to the first and third bottom plates, and the DC voltage maybe applied to the second bottom plate. The first and third bottom plates(e.g., bottom plates 130 and 132) may be formed on two sides of thesecond bottom plate (e.g., bottom plate 120), which may be formed undera central area of the top plate.

In another exemplary design, the MEMS varactor may further comprise athird bottom plate (e.g., bottom plate 132 in FIG. 1) formed under thetop plate and electrically coupled to the second terminal. In thisexemplary design, which may correspond to mode 1 in FIG. 3B, the DCvoltage may be applied to the second and third bottom plates, and theinput signal may be applied to the first bottom plate. The second andthird bottom plates (e.g., bottom plates 130 and 132) may be formed ontwo sides of the first bottom plate (e.g., bottom plate 120), which maybe formed under the central area of the top plate.

In yet another exemplary design, at least one additional first bottomplate may be formed under the top plate and electrically coupled to thefirst terminal, e.g., as shown in FIGS. 4 and 5. At least one additionalsecond bottom plate may also be formed under the top plate andelectrically coupled to the second terminal, e.g., as also shown inFIGS. 4 and 5.

In an exemplary design, the first bottom plate (which receives the inputsignal) may have a smaller area than the second plate (which receivesthe DC voltage) in order to reduce changes in capacitance due to signalswing of the input signal, e.g., as shown in FIG. 6A. In anotherexemplary design, the second bottom plate may be thicker than the firstbottom plate in order to reduce changes in capacitance due to signalswing of the input signal, e.g., as shown in FIG. 6B. In yet anotherexemplary design, bottom plates located progressively closer to thecentral area of the top plate may have progressively larger gap to thetop plate, e.g., as shown in FIG. 6D.

In another aspect, a MEMS varactor may comprise first, second and thirdplates. The first plate (e.g., plate 720 in FIG. 7 or plate 820 in FIG.8) may be electrically coupled to a first terminal, which may receive afirst DC voltage. The second plate (e.g., plate 730 in FIG. 7 or plate830 in FIG. 8) may be formed over the first plate and may beelectrically coupled to a second terminal. The third plate (e.g., plate740 in FIG. 7 or plate 840 in FIG. 8) may be formed over the secondplate and may be electrically coupled to a third terminal, which mayreceive a second DC voltage. An input signal may be passed between thefirst and second terminals. The first and second DC voltages may causethe second plate to mechanically move and vary the capacitance observedby the input signal.

In an exemplary design, the input signal may be applied to the firstterminal and capacitively passed to the second terminal, e.g., as shownin FIG. 7. In this exemplary design, the input signal may observe thecapacitance between the first and second plates. The second DC voltagemay be determined based on signal swing of the input signal. In anotherexemplary design, the input signal may be applied to the second terminaland capacitively passed to both the first and third terminals, e.g., asshown in FIG. 8. In this exemplary design, the input signal may observethe capacitance between the first and second plates plus the capacitancebetween the second and third plates.

In yet another aspect, an apparatus (e.g., a wireless communicationdevice) may comprise a filter that receives an input signal and providesan output signal. The filter may include a MEMS varactor, which maycomprise first, second and third plates. The first plate may beelectrically coupled to a first terminal, the second plate may beelectrically coupled to a second terminal, and the third plate may beelectrically coupled to a third terminal. The MEMS varactor may have avariable capacitance determined by at least one DC voltage applied to atleast one of the first, second and third terminals. The filter mayfurther comprise an inductor coupled in parallel with the MEMS varactorand forming a resonator to attenuate the input signal at a designatedfrequency.

In an exemplary design, the first and second plates may be formed on acommon layer and under the third plate, e.g., as shown in FIGS. 1 and 2.The at least one DC voltage may comprise a DC voltage applied to thefirst terminal to cause the third plate to mechanically move and varythe capacitance of the MEMS varactor. In another exemplary design, thesecond plate may be being formed over the first plate, and the thirdplate may be formed over the second plate, e.g., as shown in FIG. 7 or8. The at least one DC voltage may cause the second plate tomechanically move and vary the capacitance of the MEMS varactor.

FIG. 11 shows an exemplary design of a process 1100 for operating a MEMSvaractor. A DC voltage may be applied to a first plate of a MEMSvaractor comprising the first plate, a second plate, and a third plate(block 1112). An input signal may be applied to at least one of thefirst, second, and third plates of the MEMS varactor (block 1114). TheDC voltage may be varied to mechanically move the third plate of theMEMS varactor and vary the capacitance observed by the input signal(block 1116).

In an exemplary design, the input signal may be applied to the secondplate of the MEMS varactor. The first and second plates (e.g., plates120 and 130 in FIG. 1) may be formed under the third plate (e.g., plate150 in FIG. 1).

In another exemplary design, the input signal may be applied to thefirst plate (e.g., plate 720 in FIG. 7). A second DC voltage may beapplied to the second plate (e.g., plate 740). The third plate (e.g.,plate 730) may be formed over the first plate, and the second plate maybe formed over the third plate. The second DC voltage may be generatedbased on the signal swing of the input signal.

In yet another exemplary design, the input signal may be applied to thethird plate (e.g., plate 830 in FIG. 8). A second DC voltage may beapplied to the second plate (e.g., plate 840). The third plate may beformed over the first plate (e.g., plate 820), and the second plate maybe formed over the third plate.

The MEMS varactors described herein may be fabricated with various MEMSprocess technologies known in the art. The MEMS varactors may befabricated on a substrate (e.g., a glass or silicon substrate) and maybe encapsulated in a suitable package. A substrate with MEMS varactorsmay also be packaged together with a semiconductor IC die. The MEMSvaractors may also be fabricated on a semiconductor IC (e.g., a siliconbased CMOS IC, a GaAs or InP based compound semiconductor IC, etc.)using semiconductor process technology.

An apparatus implementing any of the MEMS varactors described herein maybe a stand-alone device or may be part of a larger device. A device maybe (i) a stand-alone IC package, (ii) a set of one or more IC packagesthat may include memory ICs for storing data and/or instructions, (iii)an RFIC such as an RF receiver (RFR) or an RF transmitter/receiver(RTR), (iv) an ASIC such as a mobile station modem (MSM), (v) a modulethat may be embedded within other devices, (vi) a receiver, cellularphone, wireless device, handset, or mobile unit, (vii) etc.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by acomputer. By way of example, and not limitation, such computer-readablemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium that can be used to carry or store desired program code inthe form of instructions or data structures and that can be accessed bya computer. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the scope of thedisclosure. Thus, the disclosure is not intended to be limited to theexamples and designs described herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A micro-electro-mechanical system (MEMS) varactor, comprising: afirst bottom plate electrically coupled to a first terminal receiving aninput signal; a second bottom plate electrically coupled to a secondterminal receiving a DC voltage; and a top plate formed over the firstand second bottom plates and electrically coupled to a third terminal,the DC voltage causing the top plate to mechanically move and varycapacitance observed by the input signal.
 2. The MEMS varactor of claim1, further comprising: a third bottom plate formed under the top plateand electrically coupled to the first terminal.
 3. The MEMS varactor ofclaim 2, the first and third bottom plates being formed on two sides ofthe second bottom plate, and the second bottom plate being formed undera central area of the top plate.
 4. The MEMS varactor of claim 1,further comprising: a third bottom plate formed under the top plate andelectrically coupled to the second terminal.
 5. The MEMS varactor ofclaim 4, the second and third bottom plates being formed on two sides ofthe first bottom plate, and the first bottom plate being formed under acentral area of the top plate.
 6. The MEMS varactor of claim 1, furthercomprising: at least one additional first bottom plate formed under thetop plate and electrically coupled to the first terminal; and at leastone additional second bottom plate formed under the top plate andelectrically coupled to the second terminal.
 7. The MEMS varactor ofclaim 1, the first bottom plate having a smaller area than the secondbottom plate to reduce changes in capacitance due to signal swing of theinput signal.
 8. The MEMS varactor of claim 1, the second bottom platebeing thicker than the first bottom plate to reduce changes incapacitance due to signal swing of the input signal.
 9. The MEMSvaractor of claim 1, the second bottom plate being thinner than thefirst bottom plate to increase capacitance, or capacitance tuning range,or both of the MEMS varactor.
 10. The MEMS varactor of claim 1, withbottom plates located progressively closer to a central area of the topplate having progressively larger gap to the top plate.
 11. The MEMSvaractor of claim 1, the input signal comprising a radio frequency (RF)signal.
 12. The MEMS varactor of claim 1, the input signal having asignal swing of more than 10 Volts.
 13. A micro-electro-mechanicalsystem (MEMS) varactor, comprising: a first plate electrically coupledto a first terminal receiving a first DC voltage; a second plate formedover the first plate and electrically coupled to a second terminal; anda third plate formed over the second plate and electrically coupled to athird terminal receiving a second DC voltage, an input signal beingpassed between the first and second terminals, and the first and secondDC voltages causing the second plate to mechanically move and varycapacitance observed by the input signal.
 14. The MEMS varactor of claim13, the input signal being applied to the first terminal andcapacitively passed to the second terminal.
 15. The MEMS varactor ofclaim 14, the input signal observing capacitance between the first andsecond plates.
 16. The MEMS varactor of claim 13, the input signal beingapplied to the second terminal and capacitively passed to both the firstand third terminals.
 17. The MEMS varactor of claim 16, the input signalobserving capacitance between the first and second plates andcapacitance between the second and third plates.
 18. The MEMS varactorof claim 13, the second DC voltage being determined based on a signalswing of the input signal.
 19. An apparatus comprising: a filter toreceive an input signal and provide an output signal, the filterincluding a micro-electro-mechanical system (MEMS) varactor comprising afirst plate electrically coupled to a first terminal, a second plateelectrically coupled to a second terminal, and a third plateelectrically coupled to a third terminal, the MEMS varactor having avariable capacitance determined by at least one DC voltage applied to atleast one of the first, second and third terminals.
 20. The apparatus ofclaim 19, the filter further comprising an inductor coupled in parallelwith the MEMS varactor and forming a resonator to attenuate the inputsignal at a designated frequency.
 21. The apparatus of claim 19, thefirst and second plates being formed on a common layer and under thethird plate, the at least one DC voltage comprising a DC voltage appliedto the first terminal and causing the third plate to mechanically moveand vary capacitance of the MEMS varactor.
 22. The apparatus of claim19, the second plate being formed over the first plate, and the thirdplate being formed over the second plate, the at least one DC voltagecausing the second plate to mechanically move and vary capacitance ofthe MEMS varactor.
 23. A method comprising: applying a DC voltage to afirst plate of a micro-electro-mechanical system (MEMS) varactorcomprising the first plate, a second plate, and a third plate; applyingan input signal to at least one of the first, second, and third platesof the MEMS varactor; and varying the DC voltage to mechanically movethe third plate of the MEMS varactor and vary capacitance observed bythe input signal.
 24. The method of claim 23, the applying the inputsignal comprises applying the input signal to the second plate of theMEMS varactor, the first and second plates being formed under the thirdplate.
 25. The method of claim 23, further comprising: applying a secondDC voltage to the second plate of the MEMS varactor, the input signalbeing applied to the first plate, the third plate being formed over thefirst plate, and the second plate being formed over the third plate. 26.The method of claim 25, further comprising: generating the second DCvoltage based on a signal swing of the input signal.
 27. The method ofclaim 23, further comprising: applying a second DC voltage to the secondplate of the MEMS varactor, the input signal being applied to the thirdplate formed over the first plate, and the second plate being formedover the third plate.
 28. An apparatus comprising: means for applying aDC voltage to a first plate of a micro-electro-mechanical system (MEMS)varactor comprising the first plate, a second plate, and a third plate;means for applying an input signal to at least one of the first, second,and third plates of the MEMS varactor; and means for varying the DCvoltage to mechanically move the third plate of the MEMS varactor andvary capacitance observed by the input signal.